Implants are used in a variety of ways in modern medical technology. Implants serve, among other things, to support blood vessels, hollow organs and duct systems (endovascular implants), for fastening and temporary fixation of tissue implants and tissue transplants, and also for orthopedic purposes, e.g., as nails, plates or screws.
Implantation of stents is one of the most effective therapeutic measures in treatment of vascular diseases. Stents assume a supporting function in the hollow organs of a patient. Stents of a traditional design, therefore, have a filigree supporting structure of metallic struts, the structure initially being in a compressed form for introduction into the body and then dilated at the site of application. One of the main areas of application of such stents is for permanent or temporary dilation of vascular occlusions and maintaining vascular patency, in particular, for dilation of occlusions (stenoses) of the coronary vessels. In addition, aneurysm stents, which provide support for damaged vascular walls, are also known.
The base body of each implant, in particular, stents, comprises an implant material. For purposes of the present disclosure, an implant material is a nonviable material that is used for an application in medicine and interacts with biological systems. The basic prerequisites for use of a material as an implant material, which comes in contact with the biological environment when used as intended, is its biological compatibility (biocompatibility). For purposes of the present disclosure, biocompatibility is the ability of a material to induce an appropriate tissue reaction in a specific application. This includes an adaptation of the chemical, physical, biological and morphological surface properties of an implant to the recipient tissue with the goal of a clinically desired interaction. The biocompatibility of the implant material also depends on the chronological course of the reaction of the biosystem in which the implant is implanted. Thus, irritations and inflammations, which can lead to tissue changes, occur in the relatively short term. Biological systems thus react in different ways, depending on the properties of the implant material. According to the reaction of the biosystem, the implant materials can be subdivided into bioactive, bioinert and degradable/absorbable materials. For the purposes of the present disclosure only degradable/absorbable metallic implant materials are of interest. The degradable/absorbable metallic implant materials are referred to hereinbelow as biocorrodible metallic materials.
The use of biocorrodible metallic materials is recommended, in particular, because the implant often must remain in the body only a short period of time to fulfill the medical purpose. Implants of permanent materials, i.e., materials that are not degraded in the body, can optionally be removed again because there may be rejection reactions in the body in the medium range and in the long range even if there is a high biocompatibility.
One approach to avoid a further surgical intervention consists of making the implant either entirely or partially of a biocorrodible metallic material. For purposes of the present disclosure, “biocorrosion” refers to processes which are due to the presence of biological media and lead to a gradual degradation of the structure made of this material. At a certain point in time, the implant, or at least the part of the implant made of the biocorrodible material, loses its mechanical integrity. The degradation products are largely absorbed by the body. In the best case, the degradation products such as magnesium, for example, even have a positive therapeutic effect on the surrounding tissue. Small quantities of unabsorbable alloy constituents are tolerable, as long as they are nontoxic.
Known biocorrodible metallic materials comprise pure iron and biocorrodible alloys of the main elements magnesium, iron, zinc, molybdenum and tungsten. Among other things, it is proposed in German Patent Application No. 197 31 021 that medical implants should be made of a metallic material having as its main component an element from the group consisting of alkali metals, alkaline earth metals, iron, zinc and aluminum. Alloys based on magnesium, iron and zinc are described as being especially suitable. Secondary constituents of the alloys may include manganese, cobalt, nickel, chromium, copper, cadmium, lead, tin, thorium, zirconium, silver, gold, palladium, platinum, silicon, calcium, lithium, aluminum, zinc and iron. Regardless of advances made in the field of biocorrodible metal alloys, the alloys known so far can be used only to a very limited extent because of their corrosion behavior. In particular, the relatively slow biocorrosion of pure iron or the known iron alloys limits their possible applications.
Traditional fields of use of molded bodies of metallic materials, in particular, iron alloys outside of medical technology usually require extensive suppression of corrosion processes. Accordingly, the purpose of most technical methods for improving the corrosion behavior is to completely inhibit corrosion processes. However, the purpose of improving the corrosion behavior of the biocorrodible iron alloys in the present disclosure is not to completely suppress corrosion processes but rather to accelerate the corrosion processes. Furthermore, toxicological aspects must also be taken into account for any application in medical technology. Finally, corrosion processes depend greatly on the medium in which the corrosion processes take place. Therefore, it is usually impossible to transfer findings about the properties of specific iron alloys obtained in the technical field under traditional nonphysiological environmental conditions to the processes in a physiological environment.
International Patent Publication No. WO 2007/082147 relates to bioerodable endoprostheses having at least two sections of metallic materials with different corrosion rates. Iron alloys with the following compositions are mentioned as examples: (i) 88 to 99.8 wt % Fe, 0.1-7 wt % Cr, 0-3.5 wt % Ni and less than 5 wt % other elements and (ii) 90 to 96 wt % Fe, 3-6 wt % Cr, 0-3 wt % Ni and 0-5 wt % other metals. When using the elements Cr and Ni in a biocorrodible material, a substantial negative effect on biocompatibility must be expected. Chromium, in particular, is among the elements having a very high toxicity potential.
Hendra Hermawan et al. describe the degradation behavior of a stent made of the Fe-35Mn alloy (Advanced Materials Research, vols. 15-17, (2007), pp. 113-116). However, this alloy has only a marginally increased corrosion. Another disadvantage is that the relatively high Mn content in a monophase structure suggests a reduced ductility.
Another disadvantage of an alloy with 35 wt % Mn is that after cooling, transformation to so-called ε-(epsilon)-martensite is no longer possible.
Another disadvantage of an alloy with 35% Mn is that a two-phase structure cannot be achieved here. However, a fine-grained two-phase structure is the basis of extremely good plasticity. One disadvantage of the loss of this multiphase property is that the austenite stability can no longer be adjusted with carbon or nitrogen.
Biocorrodible alloys of iron and carbon are also known (for example, in European Patent Application No. 0 923 389 and International Patent Publication No. WO 2007/124230). One disadvantage of these alloys is that a pure binary system of iron and carbon shows a great decline in ductility with an increase in carbon content without a comparable decline in corrosion resistance.
One feature of the present invention provides a biocorrodible iron alloy having improved corrosion behavior for an implant. This should take place, in particular, in such a way that the additional material properties that are important for processing, e.g., ductility, are not impaired.